1. Field of the Invention
The present invention relates to a patterning process using a sidewall spacer method.
2. Description of the Related Art
In recent years, as LSI progresses toward higher integration and further acceleration in speed, miniaturization of a pattern rule is required. In the light-exposure used as a general technology nowadays, resolution inherent in wavelength of a light source is approaching to its limit. In 1980s, a g-line (436 nm) or an i-line (365 nm) of a mercury lamp was used as an exposure light. As a mean for further miniaturization, shifting to a shorter wavelength of an exposing light was assumed to be effective. As a result, in a mass production process after DRAM (Dynamic Random Access Memory) with 64-megabits (0.25 μm or less of a processing dimension) in 1990s, a KrF excimer laser (248 nm), a shorter wavelength than an i-line (365 nm), was used in place of an i-line as an exposure light source.
However, in production of DRAM with an integration of 256 M, 1 G and higher which require further miniaturized process technologies (process dimension of 0.2 μm or less), a light source with a further short wavelength is required, and thus a photo lithography using an ArF excimer laser (193 nm) has been investigated seriously since about a decade ago. At first, an ArF lithography was planned to be applied to a device starting from a 180-nm node device, but a KrF excimer laser lithography lived long to a mass production of a 130-nm node device, and thus a full-fledged application of an ArF lithography will start from a 90-nm node. Further, a study of a 65-nm node device by combining with a lens having an increased NA till 0.9 is now underway. Further shortening of wavelength of an exposure light is progressing towards the next 45-nm node device, and for that an F2 lithography with a 157-nm wavelength became a candidate. However, there are many problems in an F2 lithography; an increase in cost of a scanner due to the use of a large quantity of expensive CaF2 single crystals for a projector lens, extremely poor sustainability of a soft pellicle, which leads to a change of an optical system due to introduction of a hard pellicle, a decrease in an etching resistance of a resist film, and the like. Because of these problems, it was proposed to postpone an F2 lithography and to introduce an ArF immersion lithography earlier (Proc. SPIE Vol. 4690, xxix).
In an ArF immersion lithography, a proposal is made to impregnate water between a projector lens and a wafer. A refractive index of water at 193 nm is 1.44, and therefore a pattern formation is possible even if a lens with a numerical aperture (NA) of 1.0 or more is used, and moreover, theoretically NA may be increased to near 1.44. In the beginning, deterioration of a resolution and a shift of a focus due to a change of refractive index associated with a change of water temperature were pointed out. However, the problems associated with the change in the refractive index have been solved by controlling the water temperature within 1/100° C. In addition, it was also confirmed that the effect of heat generation from a resist film by light exposure was almost insignificant. As to the concern of a pattern transcription of microbubbles in water, it was also confirmed that formation of bubbles from a resist film by exposure was insignificant if water is fully degassed.
In the early period of an immersion lithography in 1980s, a proposal was made to immerse an entire stage into water. However, a partial fill method having a nozzle of water supply and of drainage in which water is introduced only between a projector lens and a wafer in order to meet the movement of a high-speed scanner was adopted. By an immersion using water, designing of a lens with NA of 1 or higher became theoretically possible. However, there appeared a problem in it that a lens dimension in an optical system based on a conventional refractive index system becomes extraordinary large thereby leading to distortion of a lens due to its own weight. A proposal was made to design a catadioptric optical system for a more compact lens, which accelerated a speed in designing a lens having NA of 1.0 or more. Now a possibility of a 45-nm node is shown by combining a lens having NA of 1.2 or more with a super resolution technology (Proc. SPIE Vol. 5040, p. 724), and in addition, a development of a lens with NA 1.35 is underway.
As a 32-nm node lithography technique, a lithography of a vacuum ultraviolet beam (EUV) with a wavelength of 13.5 nm is known. Problems of an EUV lithography are requirements for a higher laser output power, a higher sensitivity of a resist film, a higher resolution, a lower line edge roughness (LWR), a non-defective MoSi laminate mask, a lower aberration of a reflective mirror, and the like, and thus there are mounting problems to be solved.
A maximum resolution in a water immersion lithography using a lens with NA of 1.35 is 40 to 38 nm, and there is no possibility to reach 32 nm. Accordingly, development of a material having a higher refractive index is underway to increase NA further. A limiting factor of NA in a lens is determined by a minimum refractive index among a projector lens, a liquid, and a resist film. In the case of a water immersion, a refractive index of water is the lowest as compared with a projector lens (refractive index of a synthetic quartz is 1.5) and a resist film (refractive index of a conventional methacrylate type is 1.7), and thus NA of the projector lens has been determined by a refractive index of water. Recently, a highly transparent liquid having a refractive index of 1.65 is under development. In this case, a refractive index of a projector lens made of a synthetic quartz is the lowest, and thus a material for a projector lens with a high refractive index needs to be developed. A refractive index of LUAG (Lu3Al5O12) is 2 or more, and thus it is expected as the most promising material, but has problems of a large double refraction and absorption. In addition, even though a projector lens material with a refractive index of 1.8 or more is developed, the highest NA reachable is 1.55 for a liquid with a refractive index of 1.65, and thus 32 nm may not be resolved.
To resolve 32 nm, a liquid with a refractive index of 1.8 or more is necessary. However, a material for it has not been found yet, because an absorption and a refractive index are in a trade-off relationship at the moment. In case of an alkane compound, a bridged cyclic compound is more preferable than a linear compound in order to increase a refractive index, but a cyclic compound has a problem that it cannot follow a high-speed scanning of a stage of an exposure instrument because of its high viscosity. In addition, if a liquid having a refractive index of 1.8 is developed, a minimum refractive index lies in a resist film, and therefore, a resist film with a refractive index of 1.8 or more is also needed.
Recently, a double patterning process, in which a pattern is formed by a first exposure and development, and a pattern is formed by a second exposure exactly in a space of the first pattern, is drawing an attention (Jpn. J. Appl. Phys., Vol. 33 (1994), p. 6874-6877, Part 1, No. 12B, December 1994). Many processes are proposed as the double patterning method. For example, there is a method in which a photoresist pattern with a line and space interval of 1:3 is formed by a first exposure and development, an underlying hard mask is processed by dry etching, an another hard mask is formed on it, then, by exposure and development of a photoresist film to form a line pattern in a space formed by the first exposure, and then the hard mask is dry etched to form a line-and-space pattern with a half width of the first pattern pitch. There is also another method in which a photoresist pattern with a space and line interval of 1:3 is formed by a first exposure and development, an underlying hard mask is processed by dry etching, a photoresist film is applied on it, the second space-pattern is exposed on a remaining part of the hard mask, and then the hard mask is dry etched. In both methods, hard masks are processed by two dry etching steps.
In the former methods as mentioned above, a hard mask needs to be made twice. In the latter method, only one layer of a hard mask is needed, but a trench pattern, in which a resolution is more difficult as compared with a line pattern, needs to be formed. In the latter method, a negative resist composition may be used for formation of a trench pattern. With this method, a high contrast light similar to that used to form a line by a positive pattern may be used. However, a negative resist composition has a lower dissolution contrast as compared with a positive resist composition, and thus, a negative resist composition gives a lower resolution power as compared with the case in which lines are formed by a positive resist composition when a negative resist composition is used to form the same dimension of a trench pattern. In the latter method, it may be possible to apply a thermal flow method, in which a wide trench pattern is formed by using a positive resist composition and then the trench pattern is shrunk by heating a substrate, and a RELACS method in which a water-soluble layer is coated on a trench pattern after development and then the trench is shrunk by a thermal crosslink of a resist film surface. In these methods, however, there are problems of deterioration of a proximity bias and a low throughput due to further complicated processes.
In the both former and the latter methods, two etchings are necessary in substrate processing, thereby causing problems of a lower throughput as well as a deformation and a misalignment of the pattern due to these two etchings.
To perform the etching only once, there is a method in which a negative resist composition is used in the first exposure and a positive resist composition is used in the second exposure. There is another method in which a positive resist composition is used in the first exposure and a negative resist composition dissolved in a higher alcohol having 4 or more carbon atoms and not dissolving the positive resist composition is used in the second exposure. In these methods, a resolution is deteriorated due to the use of a negative resist composition having a low resolution.
The most critical problem in the double patterning is an overlay accuracy of the first and the second patterns. A magnitude of the position displacement corresponds to variation of the line dimension. Thus, for example, to form a 32-nm line with 10% accuracy, an overlay accuracy within 3.2 nanometers is necessary. Because an overlay accuracy of a currently used scanner is about 8 nm, a substantial improvement in the accuracy is necessary.
Because of problem of the overlay accuracy of a scanner and the difficulty to divide one pattern into two, a method in which a pitch is divided into halves in a single exposure is investigated.
A method in which a pitch is divided into halves by forming films on both sides of a line pattern sidewall is proposed (J. Vac. Sci. Technol. B17(6), November/December 1999). For this sidewall spacer method, a spacer space method, in which a hard mask under the resist film and a film embedded into a space between films attached on its sidewall are used as an etching pattern, and a spacer line method, in which a film attached on a hard mask sidewall under the resist film is used as an etching pattern, are proposed (4th International Symposium on Liquid Immersion (2007), Presentation No.: PR-01, Title: Implementation of immersion lithography to NAND/CMOS device manufacturing).
These processes are proposed, but no more practicable method or art has been proposed yet under present circumstances.
As discussed above, with the progress of miniaturization of the pattern rule in recent years, a patterning process to form a further finer pattern simply and efficiency is desired.